Indeed, if you skip this you're going to have a weird "bug" some time in the next years where a customer accidentally got the ID wrong and the system accepted it.

Google Analytics either doesn't do this or got really unlucky. But some day I had to troubleshoot an issue where the data for a totally unrelated website ended up in someone's GA data set. Not just the usual spam, but millions of visits to the wrong website which had an account ID very similar to the client's.